Various types of sheets transport apparatus are known, and one such system includes a chain or belt system in which spaced grippers grip the leading edge of printed sheets being delivered thereto from a delivery drum. Apparatus of this type is described, for example, in the textbook "Einfuhrung in den Offsetdruck" ("Introduction to Offseet Printing"), by Wolfgang Walenski, 1975, page 96. As shown in this textbook, sheets are transferred from a cylinder of a printing system to a gripper chain system. Upon such transfer, a reduction in speed of movement of the sheets will occur as soon as the chain gripper system leaves the radius of the drum behind the delivery drum, or the last drum, or cylinder of the printing machine. As a consequence, the trailing end of the printed subject matter may still be located between two printing cylinders, operating at a higher linear speed than the transport drum or chain. The trailing end of the sheet, thus, will move faster than the leading edge which is in the transport chain system.
It has been found in practice that such a system may result in ghost printing at the trailing edge of the sheet. The reason, apparently, is that no tension is applied to the sheet by the cross bars, with the grippers, gripping the leading edge, and operating at a linear speed less than the circumferential speed of the printing cylinders. This undesirable effect is enhanced if the diameter of the delivery drum is less than the diameter of one of the cylinders of the printing system and, particularly, the printing or impression cylinder.
The problem is not serious in printing machines which are constructed in serial form, that is, one printing system behind another; it is, however, sometimes annoying if a chain transport apparatus is combined with a five-cylinder printing machine. In such apparatus, the quality of print, particularly at the trailing edge of the sheet, may be impaired. It appears that the reason therefor is that, after transfer of the sheet to the chain gripper system, the trailing edge of the sheet will still be located between two printing or impression lines, that is, for example between two blanket cylinders and the impression cylinder. The tension forces acting on the sheets are not applied from the leading edge, and the differential sheet travel along one of the blanket cylinders, upon being passed along the second blanket cylinder, will lead to the above referred-to ghosts, which is more noticeable as the edge of the sheet is approached.